It is important in many areas of government and business to insure that voice messages be transmitted to a remote location with a high degree of privacy. Thus, systems have previously been developed for message scrambling or encryption wherein a voice signal is divided into a plurality of sub-bands and the order of the sub-bands is then randomly varied in conjunction with random inversion of ones of the sub-bands. The reordered and inverted sub-bands are then transmitted over a communications link in an unintelligible state. Additionally, previously developed systems have divided a voice signal into a plurality of discrete time-frequency segments. The order of these segments are then randomly rearranged in order to render the voice signal unintelligible. Only a remote receiving station having a properly coded unit is able to properly reorder the frequency bands in time and space in order to render the voice signal intelligible.
Examples of such previously developed voice scrambling systems may be found in U.S. Pat. No. 1,709,901 issued to Espenschied et al on Apr. 23, 1929 and entitled "Secret Signaling System"; U.S. Pat. No. 2,411,683 issued to Guanella on Nov. 26, 1946 and entitled "Method and Arrangement for Scrambling Speech Signals"; and U.S. Pat. No. 4,020,285 issued to Branscome et al on Apr. 26, 1977 and entitled "Voice Security Method and System". Additionally, examples of such previously developed and proposed voice scrambling systems are described in a publication entitled "Final Report on Project C-66, Frequency Time Division Speech Privacy System" prepared for Division 13, Section 3 of the National Defense Research Committee of the Office of Scientific Research and Development by Bell Telephone Laboratories, Inc., dated May 29, 1943 (Contact No. OEMsr-795).
However, many previously developed voice scrambling systems have not been sufficiently secure for users of such systems. The level of security has also depended upon the complexity of the scrambling or encryption system, and it has been thought that a more complex and costly built system insured a more secure system. Therefore, a need has developed for a relatively simple encryption system which provides a high level of security. A need has also arisen for a two-dimensional scrambling system where both frequency and time dimensions are utilized in the encryption process. A need has further arisen for an encryption system in which a plurality of remote stations utilize a single encryption unit where multiple users desire to transmit secure messages.